Brake servo-unit comprising a floating element bearing offset valve seats

ABSTRACT

A pneumatic servomotor for power-assisted braking for a motor vehicle, which servomotor is of the type in which the movements of the control rod ( 38 ) can determine opening and closing of at least one axial valve ( 52 ) referred to as an “inlet” valve and which is interposed between a pressure source subjected to a pressure (P a ) greater than a first pressure (P 1 ) and a back chamber ( 18 ), and at least one axial valve ( 50 ) referred to as a “balancing” valve and which is interposed between the front chamber ( 16 ) and the back chamber ( 18 ), for actuating a moving partition ( 14 ) interposed between the front chamber ( 16 ) and the back chamber ( 18 ), said servomotor being characterized in that it includes a one-piece floating tubular element ( 76 ) which has axially offset transverse faces ( 78, 80 ) that are provided with first sealing elements ( 82, 84 ) for sealing the axial inlet and balancing valves ( 50, 52 ) that the control rod ( 38 ) can actuate via a dish-shaped washer ( 49 ) carrying two springs ( 40, 122 ) of different stiffnesses.

The invention relates to a pneumatic servomotor for power-assistedbraking for a motor vehicle.

The invention relates more particularly to a pneumatic servomotor forpower-assisted braking for a motor vehicle, which servomotor is: of thetype having a rigid casing inside which a moving transverse partition ismounted to define in leaktight manner a front chamber subjected to afirst pressure, and a back chamber subjected to a second pressurevarying between the first pressure and a pressure that is greater thanthe first pressure, which partition can drive an actuating rod foractuating a master cylinder associated with the servomotor via areaction disk, and is returned resiliently by first return means; of thetype having a control rod moving inside the piston selectively as afunction of an axial inlet force exerted forwards against a return forceapplied to the rod by second return means; of the type in which themovements of the control rod can determine opening and closing of atleast one axial valve referred to as an “inlet” valve and which isinterposed between a pressure source subjected to the pressure greaterthan the first pressure and the back chamber, and at least one axialvalve referred to as a “balancing” valve and which is interposed betweenthe front chamber and the back chamber, for actuating theabove-described moving partition; and of the type in which a plungerpassing through the moving partition and secured to the end of thecontrol rod can drive the actuating rod of the master cylinder directly,via the reaction disk.

Numerous examples of conventional servomotors of this type are known.

In such a servomotor, a piston is secured to the moving wall into whichit is fitted, for example, and it receives the inlet and balancingvalves. In addition, the balancing valve and the inlet valve are part ofa single three-port valve in which a common seat is constituted by ashoulder front face of a moving element which is resiliently returnedagainst a collar on the plunger and against a collar on the piston, fromwhich collars it can be selectively separated to open the balancingvalve or the inlet valve.

Thus, a substantially radial duct that passes through the piston andthat opens out into the front chamber, and a substantially axial ductwhich opens out to the outside of the servomotor are selectively putinto communication via a respective one of valves with a radial ductthat passes through the piston and that opens out in the back chamber tomaintain or to reduce to zero a pressure difference between the back andthe front chambers, and to cause the moving partition to move.

Such a design suffers from the drawback of requiring ducts to be formedin the piston with complex shapes, which disturbs the flow of air. As aresult, a servomotor generally has response times that are relativelyhigh, and is noisy.

In addition, such a design is particularly costly to implement becauseit requires a piston to be used whose internal shapes, serving to formthe ducts, can be formed only by performing a machining method.

The invention proposes a design making it possible to remedy thosedrawbacks, and in which the ducts are axial and are part of differenttubular elements of the servomotor. This design makes it possible tofacilitate air flow through the valves, thereby making it possible toreduce the response times of the servomotor, while also guaranteeingthat it operates quietly. In addition, this novel design makes itpossible to make most of the elements of the servomotor by implementinga stamping method, which is less costly than conventional machining ormolding methods.

To this end, the invention provides a servomotor of the above-describedtype;

-   -   characterized in that it includes:        -   a one-piece floating tubular element interposed            substantially radially between the plunger and the piston,            which element is mounted to move axially, is returned            resiliently towards the moving partition by third return            means, and has axially offset transverse faces that are            provided with first sealing elements for sealing the axial            inlet and balancing valves;        -   a complementary transverse second sealing element for            sealing the inlet axial valve, which element is carried by            the back end of the plunger; and        -   a complementary transverse second sealing element for            sealing the balancing axial valve, which element is provided            on at least a portion of the back face of the moving            partition;    -   and in that the back end of the plunger is provided with a        dish-shaped washer which includes the complementary transverse        second sealing element for sealing the inlet axial valve, and        which receives in abutment the ends of two coaxial springs,        namely a first spring of high stiffness whose other end is in        abutment against the moving partition, and which forms the        second return means associated with the control rod, and a        second spring of low stiffness whose other end is in abutment        against a shoulder face of the tubular element, and which forms        the third return means associated with said tubular element and        which enables said tubular element to be urged to close the        balancing valve.

According to other characteristics of the invention:

-   -   the floating tubular element comprises:        -   a front tubular segment which is mounted in leaktight manner            around a tubular bearing on the moving partition, which            bearing slidably receives the plunger whose front end that            carries the first sealing element of the balancing valve            facing the second sealing element of the balancing valve            that is carried by the moving partition is disposed radially            outside at least one communication hole passing through the            partition;        -   an intermediate tubular segment which communicates with the            back chamber; and        -   a back tubular segment which is of diameter smaller than the            diameter of the intermediate segment, which is slidably            guided in leaktight manner in a back tubular segment of the            casing that forms an inlet duct fed by the pressure source            subjected to the pressure greater than the first pressure,            and whose front transverse face connecting to the            intermediate segment carries the second sealing element of            the inlet valve facing the dish-shaped washer of the            plunger.    -   the first sealing element of the balancing valve is constituted        by a gasket which is received in the front segment of the        floating tubular element;    -   the second sealing element of the balancing valve is constituted        by a portion of the back face of the moving partition;    -   the first sealing element of the inlet valve is constituted by a        front transverse face of a wall interconnecting the intermediate        segment and the back segment of the floating tubular element;        and    -   the second sealing element of the inlet valve is constituted by        a gasket which is carried by a back face of the dish-shaped        washer that is secured to the plunger;    -   the tubular bearing for guiding the plunger, which bearing is        carried by the moving partition, extends axially from the back        face of the moving partition, and the moving partition is        provided with a plurality of holes distributed angularly through        the transverse partition around the junction where its tubular        bearing meets its back face;    -   the first spring is disposed inside the second spring, and the        two springs are mounted inside the tubular element;    -   the first spring is interposed between a front face of the        dish-shaped washer and a shoulder face of the front tubular        bearing, and in that the second spring is interposed between the        front face of the dish-shaped washer and front end transverse        faces of opposite longitudinal slots in the intermediate segment        of the tubular element, which slots make it possible for        communication to be established with the back chamber;    -   the first spring is disposed outside the second spring, and the        two springs are mounted outside the tubular element via an        annular holding ring having at least two diametrically opposite        radial arms passing through associated slots in the tubular        element to come to bear on the dish-shaped washer;    -   the first spring is interposed between a shoulder face of the        annular ring and a bearing cup which surrounds the tubular        element and which is secured to the moving partition, and the        second spring is interposed between an end face of the tubular        annular ring and a shoulder transverse face of the tubular        element that defines the front segment and the intermediate        segment thereof;    -   the back segment of the tubular element is mounted in leaktight        manner in the back segment of the casing via a lip gasket;    -   the plunger is mounted to slide in the tubular bearing of the        moving partition via a slidably mounted tubular element having a        front collar which is disposed facing the reaction disk and        which can be driven by a collar on the plunger for a force of        determined magnitude to transmit the reaction force from the        actuating rod of the master cylinder in part to the control rod        of the servomotor and in part to the moving partition; and    -   the moving partition, the floating tubular element, and the        dish-shaped washer secured to the plunger are made by        implementing cutting and stamping methods.

Other characteristics and advantages of the invention appear on readingthe following detailed description which is made easier to understand byreferring to the accompanying drawings, in which:

FIG. 1 is an overall axial section view of a servobrake having a priorart pneumatic servomotor for power-assisted braking;

FIG. 2 is a detail view of the servomotor of FIG. 1;

FIG. 3 a is a perspective view of the elements inside the casing of afirst embodiment of a servomotor of the invention;

FIG. 3 b is a perspective view of the elements inside the casing of asecond embodiment of a servomotor of the invention;

FIGS. 4 a and 4 b are detail views in axial section through servomotorsassociated respectively with the first and with the second embodimentsof the invention, the plunger being at rest, the balancing valve beingopen, and the inlet valve being closed;

FIGS. 5 a and 5 b are detail views in section through servomotorsassociated respectively with the first and with the second embodimentsof the invention, the plunger being actuated and the balancing valve andthe inlet valve being closed;

FIGS. 6 a and 6 b are detail views in section through servomotorsassociated respectively with the first and with the second embodimentsof the invention, the plunger being actuated, the balancing valve beingclosed, and the inlet valve being open; and

FIGS. 7 a and 7 b are detail views in section through servomotorsassociated respectively with the first and with the second embodimentsof the invention, the plunger being released, the balancing valve beingre-opened, and the inlet valve being re-closed.

In the following description, like references designate parts that areidentical or that have similar functions.

By convention, the terms “front”, “back”, “top”, and “bottom”respectively designate elements or positions disposed respectivelytowards the left, the right, the top, and the bottom of FIGS. 1 to 7 b.

FIG. 1 shows a servobrake 10 including a conventional pneumaticservomotor 11 for power-assisted braking for a motor vehicle. Theservomotor 11 serves to actuate a master cylinder 13 for braking thevehicle.

In known manner, the pneumatic servomotor 10 has a rigid casing 12inside which a transverse partition 14 is mounted that defines inleaktight manner a front chamber 16 subjected to a first pressure “P₁”,and a back chamber 18 subjected to a second pressure “P₂”. The secondpressure “P₂” can vary between the value of the pressure “P₁” and thevalue of a pressure “P_(a)” that is greater than the pressure “P₁” so asto cause the partition 14 to move, which partition can, as explainedbelow, drive an actuating rod 28 for actuating the master cylinder 13.

More particularly, the pressure “P₁” corresponds in particular to apressure delivered by a suction source of the vehicle. When the engineis a spark-ignition engine, the suction pressure “P₁” is, for example,delivered by an intake manifold of the engine of the vehicle, and whenthe engine is a compression-ignition engine of the “diesel” type, thesuction pressure “P₁” is, for example, delivered by a vacuum pump of thevehicle.

The front chamber 16 is connected to the suction source of the vehiclevia a suction duct 20 which opens out into the casing 12.

The pneumatic servomotor 11 includes a moving piston 22 having an axis Aand that is secured to the moving partition 14. For example, the movingpiston 22 is fitted through the moving partition 14.

Inside the casing 12, the moving partition 14 is returned resiliently byfirst return means constituted by a return spring 24 which abuts againstthe casing 12 and against a front face 26 of the moving piston 22. Thefront face 26 of the moving piston 22 has a front cylindrical bearing 27which is organized to face a reaction disk 32 which is received in adish-shaped washer 30 that is secured to or integral with the actuatingrod 28 for actuating the master cylinder 13.

A control rod 38 which is, for example, connected to a brake pedal ofthe vehicle via a coupling sleeve 41, can move inside a casing 12, andmore particularly inside the moving piston 22, selectively as a functionof an axial inlet force exerted forwards on the sleeve 41. The actuatingforce is exerted against a return force applied to the rod 38 by secondreturn means, in particular a return spring 40 which is interposedbetween the moving piston 22 and the control rod 38.

The front end of the control rod 38, which end is opposite from thesleeve 41, is shaped in the form of a ball 42 and it is received in acomplementary socket 44 in a plunger 46 that is substantiallycylindrical and that is mounted to slide inside the moving piston 22.

As shown more particularly in FIG. 2, the movements of the control rod38 can determine opening and closing of at least one axial valve 52referred to as an “inlet” valve and which is interposed between apressure source subjected to the pressure “P_(a)” greater than the firstpressure “P₁” and the back chamber 18, and at least one axial valve 50referred to as a “balancing” valve and which is interposed between thefront chamber 16 and the back chamber 18, for actuating theabove-described moving partition.

In known manner, and in a manner not limiting the invention, thepressure source subjected to the pressure “P_(a)” corresponds to theambient atmospheric pressure. To this end, the valve 52 opens outdirectly into a back end 54 of the body of the piston 22 forming anaxial duct which is open to the ambient air.

In conventional manner, the balancing valve 50 and the inlet valve 52are part of a single valve 56 having three ports and a common seat 58constituted by a shoulder front face of a moving annular element 60which is urged resiliently by a return spring 62 against a collar 64 ofthe plunger 46 and against a collar 66 of the piston 22.

Thus, a substantially radial duct 68 which passes through the piston 22and which opens out into the front chamber 16, or else the substantiallyaxial duct 54 which opens out to the outside of the servomotor 11, can,via a guide bore 72 for guiding the plunger and via a groove 70 formedin said bore, be put selectively into communication with a radial duct74 that passes through the piston 22 and that opens out into the backchamber 18 for the purpose of maintaining or reducing to zero a pressuredifference between the front chamber 16 and the back chamber 18, andthereby causing the moving partition 14 to move.

The inlet valve 52 and the balancing valve 50 are resiliently urged intoa position in which the inlet valve 52 is closed and the balancing valve50 is open by the second return means constituted by the spring 40 forreturning the control member 38.

Such a design is particularly costly to implement because it requires apiston to be used whose inside shapes, which serve to form the ducts 68and 74, can be formed only by a machining method.

In order to remedy that drawback, the invention proposes a servomotor 11of the above-described type in which the inlet duct and the balancingduct are formed in tubular elements of the servomotor 11.

To this end, as shown in FIG. 3 et seq., the servomotor 11 does not havea piston, and it includes:

-   -   a one-piece floating tubular element 76 interposed substantially        radially between the plunger 46 and the casing 12, which element        is mounted to move axially and is returned resiliently towards        the moving partition 14 by third return means whose axially        offset transverse faces 78, 80 are provided with first sealing        elements 82, 84 for sealing the axial inlet and balancing valves        50, 52;    -   a complementary transverse second sealing element 86 for sealing        the inlet axial valve 52, which element is carried by the back        end 45 of the plunger 46; and    -   a complementary transverse second sealing element 88 for sealing        the balancing axial valve 50, which element is provided on at        least a portion of the back face 90 of the moving partition 14.

In addition, the back end 45 of the plunger 46 is provided with adish-shaped washer 49 which includes the complementary transverse secondsealing element 86 for sealing the inlet axial valve 52, and whichreceives in abutment the ends 41, 123 of two concentric springs 40, 122,namely a first spring 40 of high stiffness whose other end 39 is inabutment against the moving partition 14, and which forms the secondreturn means associated with the control rod 38, and a second spring 122of low stiffness whose other end 121 is in abutment against a shoulderface of the tubular element 76, and which forms the third return meansassociated with said tubular element 76 and which enables said tubularelement to be urged to close the balancing valve 50.

As explained below, the springs 40 and 122 can take up either of twoparticular configurations, each of which is associated with a particularembodiment of the invention.

In both embodiments of the invention, the floating tubular element 76has a tubular front segment 100 which is mounted in leaktight manneraround a tubular bearing 98 of the moving partition 14 that slidablyreceives the plunger 46. The front end 102 of the tubular front segment100 carries the first sealing element 84 for sealing the balancing valve50 facing the second sealing element 88 for sealing the balancing valve50 that is carried by the moving partition 14. The front end 102 of thefront segment 100 is disposed-radially outside at least onecommunication hole 106 passing through the partition 14.

In the invention, the floating tubular element 76 also has anintermediate tubular segment 94 which communicates with the back chamber18 via slots 105.

Finally, the floating tubular element 76 has a back tubular segment 108,of diameter smaller than the diameter of the intermediate segment 94,which is slidably guided in leaktight manner in a back tubular segment109 of the casing 12.

In particular, the back tubular segment 108 is mounted to slide througha lip gasket 113 carried by the back tubular segment 109 of the casing12.

Advantageously, the back tubular segment 108 forms an inlet duct fed bythe pressure source subjected to the pressure “P_(a)” greater than thefirst pressure “P₁”.

Advantageously, a front transverse face 78 connecting to theintermediate segment 94 carries the second sealing element 82 of theinlet valve 52 facing the dish-shaped washer 49 of the plunger 46.

More particularly, the first sealing element 82 of the inlet valve 52 isconstituted merely by the transverse face 108 interconnecting theintermediate segment 94 and the back segment 108 of the floating tubularelement 76, and the second sealing element 86 of the inlet valve 52 isconstituted by a gasket which is carried by a back face 87 of thedish-shaped washer 49 secured to the plunger 46. Preferably, the gasket87 is fitted into and/or bonded with adhesive into a groove provided inthe back face 87 of the plunger 49.

In addition, the first sealing element 84 of the balancing valve 50 isconstituted by a gasket which is fitted into and/or bonded with adhesiveinto a bore 103 in the end 102 of the front segment 100 of the floatingtubular element 76, and the second sealing element 88 of the balancingvalve 50 is constituted by a portion of the back face 90 of the movingpartition 14.

The invention is not limited to these provisions, and the gaskets can becarried respectively by the transverse face 78 interconnecting theintermediate segment 94 and the back segment 108, and by the back face90 of the moving partition 14.

The tubular bearing 98 for guiding the plunger 46, which bearing iscarried by the moving partition 14, extends axially from the back face90 of the moving partition 14 with which it is preferably integrallyformed.

More particularly, the plunger 46 is mounted to slide in the tubularbearing 98 of the moving partition 14 via a tubular element 101 which ismounted to slide in a bore 99 in the tubular bearing 98, and which has afront collar 107 disposed facing the reaction disk 32. The tubularelement 101 can be urged by a collar 109 of the plunger 46 whensubjected to a force of determined magnitude so as to transmit thereaction force from the rod for actuating the master cylinder in part tothe rod 38 for controlling the servomotor, and in part to the movingpartition 14.

Since such a tubular element 101 is widely known from the state of theart, it is not described any more explicitly in the description below.

In this configuration, the front segment 100 slides in leaktight manneron said tubular bearing 98 via a lip 85 of the gasket 84. In order toenable the balancing valve 50 to interrupt or to allow the communicationbetween the front chamber 16 and the back chamber 18, the movingpartition 14 is provided with plurality of holes 106 distributedangularly through the transverse partition 14 around the junction whereits tubular bearing 98 meets its back face 90. In addition, thisconfiguration makes it possible to guarantee that air flows as well aspossible through the moving partition 14 when the balancing valve 50 isopen.

In a first embodiment of the invention shown in FIGS. 3 a to 7 a, thefirst spring 40, which is of high stiffness, is disposed outside thesecond spring 122, which is of low stiffness. In addition, both of thesprings 40 and 122 are mounted outside the tubular element 76 inabutment against the dish-shaped washer 49 via an annular holding ring130 having at least two radial arms 132 diametrically opposite eachother passing through associated slots in the tubular element 76 forbearing against the dish-shaped washer 49.

In particular, the annular ring 130 is provided with two diametricallyopposite arms 132 which pass through the slots 105 in the intermediatesegment 94 of the floating tubular element 76.

More particularly, the first spring 40 is interposed between a shoulderface 134 of the annular ring 130 and a bearing cup 136 which surroundsthe front segment 100 of the tubular element 76 and which is in reactionagainst the moving partition 74, and the second spring 122 is interposedbetween an end face 138 of the tubular annular ring and a shoulder backtransverse face 140 of the tubular element 76, which face is disposedbehind the wall defining the front segment 100 and the intermediatesegment 94 thereof.

In a first embodiment of the invention, which is shown in FIGS. 3 b to 7b, the first spring 40 is disposed inside the second spring 122, and thetwo springs 40, 122 are mounted inside the tubular element 76.

More particularly, the first spring is interposed between a front face142 of the dish-shaped washer 49 and a shoulder face 144 of the fronttubular bearing 98, and the second spring 122 is interposed between thefront face 142 of the dish-shaped washer 49 and front end transversefaces 146 of the opposite longitudinal slots 105 in the intermediatesegment 94 of the tubular element 76, which slots make it possible forcommunication to be established with the back chamber 18.

A particular advantage of both embodiments of the invention is that themoving partition 14, the floating tubular element 76, and thedish-shaped washer 49 secured to the plunger 46 are preferably made byimplementing cutting and stamping methods. In the first embodiment ofthe invention, the ring 130 is also preferably made by implementingcutting and stamping methods. This configuration is particularlyadvantageous because it enables the servomotor 11 to be made withextremely low manufacturing costs.

In addition, in both of the embodiments of the invention, the movingpartition 14 and the tubular element 76 have simple tubular shapes, airflow is facilitated through valves 50 and 52, which makes it possible toreduce the response time of the servomotor 11 while also guaranteeingthat it operates quietly.

In this configuration, the internal elements of the servomotor 11 cantake up four different configurations which are shown respectively inFIGS. 4 a to 7 a, and 4 b to 7 b.

As shown in FIGS. 4 a and 4 b, the control rod 38 can take up a restposition. In this position, the air inlet valve 52 is closed and thebalancing valve 50 is open, so that the front chamber 16 and the backchamber 18 are subjected to the same suction pressure “P₁”. Thepartition 14 is thus stationary. The control rod 38 and the plunger 46are held in their rest position by the spring 40 which exerts a returnforce on the dish-shaped washer 49.

Starting from this position, moving the control rod 38 forwards causesthe plunger 46 and the dish-shaped washer 49 that is connected to it tomove forwards by compressing the spring 40, as shown in FIGS. 5 a and 5b.

In the first embodiment, the dish-shaped washer 49 pushes the tubularring 130 for holding the first and second springs 40, 122. Since thefirst spring 40 is of high stiffness, it compresses while the tubularbushing 130 drives the tubular element 76 via the spring 122 of lowstiffness until the gasket 84 comes into contact with the portion of theback face 90 of the partition 14, thereby causing the balancing valve 50to close as shown in FIG. 5 a.

In the second embodiment, since the second spring 40 is of highstiffness, it compresses while the dish-shaped washer 49 drives thetubular element 76 via the spring 122 of low stiffness until the gasket84 comes into contact with the portion of the back face 90 of thepartition 14, thereby causing the balancing valve 50 to close, as shownin FIG. 5 b.

In the first embodiment, by continuing to advance, the control rod 38,actuating the plunger 46, then compresses the spring 122 via the ring130, which causes the dish-shaped washer 49 and the associated gasket 86to lift off the transverse face 78 interconnecting the intermediatesegment 94 and the back segment 108 of the element 76 forming the firstsealing element 82 of the inlet valve 52, as shown in FIG. 6 a.

In the second embodiment, by continuing to advance, the control rod 38,actuating the plunger 46, directly compresses the spring 122, whichcauses the dish-shaped washer 49 and the associated gasket 86 to liftoff from the transverse face 78 interconnecting the intermediate segment94 and the back segment 108 of the element 76 forming the first sealingelement 82 of the inlet valve 52, as shown in FIG. 6 b.

In both embodiments, air at atmospheric pressure then penetrates intothe back chamber 18 and pushes away the moving partition 14.

If the control rod 38 is released, as shown in FIGS. 7 a and 7 b, thefirst spring 40 relaxing causes the plunger 46 to return towards itsrest position. While it is returning, the plunger 46 causes thedish-shaped washer 49 to come back into contact with the transverse face78 interconnecting the intermediate segment 94 and the back segment 108of the element 76, thereby closing the inlet valve 52 and then causingthe tubular element 76 to return, and therefore causing the balancingvalve 50 to re-open. As a result, the air at atmospheric pressure “Pa”contained in the back pressure chamber 18 flows into the front pressurechamber 14 which is subjected to the suction pressure “P₁”.

The invention thus makes it possible to provide a servomotor that offersa short response time and that operates particularly quietly, and that,in addition, uses a stamped tubular design to a large extent, enablingit to be manufactured at lower cost.

1. A pneumatic servomotor (11) for power-assisted braking for a motorvehicle, which servomotor is: of the type having a rigid casing (12)inside which a moving transverse partition (14) is mounted to define inleaktight manner a front chamber (16) subjected to a first pressure(P₁), and a back chamber (18) subjected to a second pressure (P₂)varying between the first pressure (P₁) and a pressure (P_(a)) that isgreater than the first pressure (P₁), which partition can drive anactuating rod (28) for actuating a master cylinder (13) associated withthe servomotor (11) via a reaction disk (32), and is returnedresiliently by first return means; of the type having a control rod (38)moving inside the casing (12) selectively as a function of an axialinlet force exerted forwards against a return force applied to the rod(38) by second return means; of the type in which the movements of thecontrol rod (38) can determine opening and closing of at least one axialvalve (52) referred to as an “inlet” valve and which is interposedbetween a pressure source subjected to the pressure (P_(a)) greater thanthe first pressure (P₁) and the back chamber (18), and at least oneaxial valve (50) referred to as a “balancing” valve and which isinterposed between the front chamber (16) and the back chamber (18), foractuating the above-described moving partition (14); and of the type inwhich a plunger (46) passing through the moving partition (14) andsecured to the end of the control rod (38) can drive the actuating rod(28) of the master cylinder (13) directly, via the reaction disk (32);said servomotor being characterized in that it includes: a one-piecefloating tubular element (76) interposed substantially radially betweenthe plunger (46) and the casing (12), which element is mounted to moveaxially, is returned resiliently towards the moving partition (14) bythird return means, and has axially offset transverse faces (78, 80)that are provided with first sealing elements (82, 84) for sealing theaxial inlet and balancing valves (50, 52); a complementary transversesecond sealing element (86) for sealing the inlet axial valve (52),which element is carried by the back end of the plunger (46); and acomplementary transverse second sealing element (88) for sealing thebalancing axial valve (50), which element is provided on at least aportion of the back face (90) of the moving partition (14); and in thatthe back end (45) of the plunger (46) is provided with a dish-shapedwasher (49) which includes the complementary transverse second sealingelement (86) for sealing the inlet axial valve (52), and which receivesin abutment the ends (41, 123) of two coaxial springs (40, 122), namelya first spring (40) of high stiffness whose other end (39) is inabutment against the moving partition (14), and which forms the secondreturn means associated with the control rod (38), and a second spring(122) of low stiffness whose other end (121) is in abutment against ashoulder face of the tubular element (76), and which forms the thirdreturn means associated with said tubular element (76) and which enablessaid tubular element to be urged to close the balancing valve (50).
 2. Apneumatic servomotor (11) according to any preceding claim,characterized in that the floating tubular element (76) comprises: afront tubular segment (100) which is mounted in leaktight manner arounda tubular bearing (98) on the moving partition (14), which bearingslidably receives the plunger (46) whose front end (102) that carriesthe first sealing element (84) of the balancing valve (50) facing thesecond sealing element (88) of the balancing valve (50) that is carriedby the moving partition (14) is disposed radially outside at least onecommunication hole (106) passing through the partition (14); anintermediate tubular segment (94) which communicates with the backchamber (18); and a back tubular segment (108) which is of diametersmaller than the diameter of the intermediate segment (94), which isslidably guided in leaktight manner in a back tubular segment (109) ofthe casing (12) that forms an inlet duct fed by the pressure sourcesubjected to the pressure (P_(a)) greater than the first pressure (P₁),and whose front transverse face (78) connecting to the intermediatesegment (94) carries the second sealing element (82) of the inlet valve(52) facing the dish-shaped washer (49) of the plunger (46).
 3. Aservomotor (11) according to any preceding claim, characterized in that:the first sealing element (84) of the balancing valve (50) isconstituted by a gasket which is received in the front segment (100) ofthe floating tubular element (76); the second sealing element (88) ofthe balancing valve (50) is constituted by a portion of the back face(90) of the moving partition (14); the first sealing element (82) of theinlet valve (52) is constituted by a front transverse face (78) of awall interconnecting the intermediate segment (94) and the back segment(108) of the floating tubular element (76); and the second sealingelement (86) of the inlet valve (52) is constituted by a gasket which iscarried by a back face (87) of the dish-shaped washer (49) that issecured to the plunger (46).
 4. A pneumatic servomotor (11) according tothe preceding claim, characterized in that the tubular bearing (98) forguiding the plunger, which bearing is carried by the moving partition(14) extends axially from the back face (90) of the moving partition(14), and in that the moving partition (14) is provided with a pluralityof holes (106) distributed angularly through the transverse partition(14) around the junction where its tubular bearing (98) meets its backface (90).
 5. A servomotor (11) according to any one of claims 1 to 4,characterized in that the first spring (40) is disposed inside thesecond spring (122), and in that the two springs are mounted inside thetubular element (76).
 6. A servomotor (11) according to the precedingclaim taken in combination with claim 2, characterized in that the firstspring (40) is interposed between a front face (142) of the dish-shapedwasher (49) and a shoulder face (144) of the front tubular bearing (98),and in that the second spring (122) is interposed between the front face(142) of the dish-shaped washer (49) and front end transverse faces(146) of opposite longitudinal slots in the intermediate segment (94) ofthe tubular element (76), which slots make it possible for communicationto be established with the back chamber (18).
 7. A servomotor (11)according to any one of claims 1 to 4, characterized in that the firstspring (40) is disposed outside the second spring (122), and in that thetwo springs (40, 122) are mounted outside the tubular element (76) inabutment against the dish-shaped washer (49) via an annular holding ring(130) having at least two diametrically opposite radial arms (132)passing through associated slots (105) in the tubular element (76) tocome to bear on the dish-shaped washer (49).
 8. A servomotor (11)according to the preceding claim taken in combination with claim 2,characterized in that the first spring (40) is interposed between ashoulder face (134) of the annular ring (130) and a bearing cup (136)which surrounds the tubular element (76) and which is secured to themoving partition (14), and in that the second spring (122) is interposedbetween an end face (138) of the tubular annular ring and a shouldertransverse face (140) of the tubular element (76) that defines the frontsegment (100) and the intermediate segment (94) thereof.
 9. A servomotor(11) according to any one of claims 2 to 8, characterized in that theback segment (108) of the tubular element (76) is mounted in leaktightmanner in the back segment (109) of the casing (12) via a lip gasket(113).
 10. A servomotor (11) according to any one of claims 2 to 9,characterized in that the plunger (46) is mounted to slide in thetubular bearing (98) of the moving partition via a slidably mountedtubular element (101) having a front collar (107) which is disposedfacing the reaction disk (32) and which can be driven by a collar (111)on the plunger (46) for a force of determined magnitude to transmit thereaction force from the actuating rod (38) of the master cylinder inpart to the control rod (38) of the servomotor and in part to the movingpartition (14).
 11. A servomotor (11) according to claims 2 to 10 takenin combination, said servomotor being characterized in that the movingpartition (14), the floating tubular element (76), and the dish-shapedwasher (49) secured to the plunger (46) are made by implementing cuttingand stamping methods.